In general, a liquid crystal display device has a structure where a liquid crystal material is sandwiched between two transparent electrode substrates, and the liquid crystal material is oriented in a direction different by 90-degree on the respective transparent electrode substrates. When a voltage is applied between the transparent electrode substrates, the liquid crystal orientation is changed to make a display. This liquid crystal display mode is referred to as a twisted nematic mode and is used in a very wide field because a structure of the liquid crystal display device is simple and a relatively favorable display characteristic is obtained.
In recent years, many fields requires a very wide viewing angle characteristic, which cannot be accomplished in the liquid crystal display device of the twisted nematic mode. For the liquid crystal display device, a liquid crystal display device of a horizontal electric field type referred to as an in-plane switching mode or fringe field switching is applied.
In the twisted nematic mode of the background art, the liquid crystal material is enabled to respond by the voltage applied between the two transparent electrode substrate and substantially perpendicular to the electrode substrate surfaces. In contrast, in the above horizontal electric field type, the liquid crystal material is enabled to respond by an electric field generated between a pair of electrodes formed on one electrode substrate and substantially parallel with the substrate surfaces. Since the liquid crystal material does not rise relative to an observer and only the orientation direction thereof is changed in the substrate surfaces, it is possible to accomplish a very wide viewing angle characteristic.
A driving principle of the liquid crystal display device of a horizontal electric field type is briefly described. The liquid crystal material is sandwiched between the two transparent substrates and is then oriented to be parallel with the substrate surfaces. A pair of electrodes is formed on the same transparent substrate and generates an electric field substantially parallel with the substrate surfaces. An orientation of the liquid crystal material at a state where no voltage is applied is referred to as an initial orientation, and a liquid crystal orientation direction at that time is referred to as an initial orientation direction. Regarding polarization plates adhered to a front side and a back side of the liquid crystal display device, one polarization plate is adhered so that a transmission axis thereof coincides with the initial orientation direction of the liquid crystal material, and the other polarization plate is adhered so that a transmission axis thereof is orthogonal to (a direction orthogonal to the transmission axis of the one polarization plate) the initial orientation direction of the liquid crystal material.
In the liquid crystal display device manufactured in this way, the light illuminated from the back side penetrates the polarization plate on the back side and is incident on the liquid crystal material with maintaining a straight polarization state. Here, it is assumed that the liquid crystal material is oriented in the initial orientation direction because a voltage is not applied thereto, and is orthogonal to the transmission axis direction of the polarization plate on the back side. At this time, the incident light penetrates the liquid crystal material without being influenced by the liquid crystal material and reaches the polarization plate on the front side. Since the polarization plate on the front side is orthogonal to the polarization direction of the traveling light, the light cannot penetrate the same. That is, at the initial orientation state where a voltage is not applied between the two electrodes, the liquid crystal display device makes a black display.
When a voltage is applied between the two electrodes of the liquid crystal display device and a horizontal electric field substantially parallel with the transparent substrate surfaces is thus generated, the liquid crystal material changes the orientation thereof from the initial orientation direction towards the electric field direction. At this state, the light incident from the back side of the liquid crystal display device penetrates the polarization plate and reaches the liquid crystal material and the polarization state thereof is changed during the penetration through the liquid crystal material of which the liquid crystal orientation direction is changed due to the horizontal electric field. Therefore, the light can penetrate the polarization plate on the front side. That is, when the voltage is applied between the two electrodes, the liquid crystal display device makes a white display.
As described above, in the liquid crystal display device of a horizontal electric field type, the adhesion direction of the polarization plate has an influence on the polarization state of the incident light, thereby highly influencing the white and black display states of the liquid crystal display device. Therefore, when the adhesion direction of the polarization plate becomes non-uniform in a process of manufacturing the liquid crystal display device and in a process of manufacturing the polarization plate, it is regarded as a non-uniformity of the display state. Particularly, when the black display state becomes non-uniform, a large non-uniformity is caused in a contrast value or a viewing angle characteristic calculated on the basis of the contrast value. One of the non-uniformity causes of the adhesion direction of the polarization plate is a precision problem in a manufacturing process of a polarization plate maker. That is, the non-uniformity in an angle caused when starting to cur a polarization plate into a desired size is one cause. Therefore, a method of measuring transmission axis directions of polarization plates manufactured in a polarization plate maker after purchasing the same, correcting a deviation of the transmission axis direction one plate-by-one plate and then adhering the polarization plates has been proposed (for example, refer to JP-A-2003-107452).